If the products you add to your store automatically show up as “sold out” on your store, you very likely have to adjust your Inventory Location settings on Shopify. Please note: Modalyst, unlike Oberlo, is not yet available as an Inventory Location on Shopify. This does NOT create a problem tracking your inventory or fulfilling orders.
Go to Settings > Locations
Select one of your Locations
Check Fulfill online orders from this location
Click View Inventory at this Location
Confirm that the inventory from Modalyst is assigned to the Location you selected
Select one of your Locations (it does not have to be the location noted as your Shipping Origin). Some users may have multiple locations available. For more information about setting up locations on Shopify, please click here.
Confirm that the inventory from Modalyst is assigned to the Location you selected
Source: HERE
From The4
Hope you success!!!